Wednesday, October 30, 2013

New paper finds the subarctic was much warmer than today during the early Holocene ~10000 - 5000 years ago

A paper published today in Quaternary Research reconstructs temperatures in subarctic Finland over the past 11,000 years and finds 4 more non-hockey-sticks with temperatures during the early Holocene ~10000 - 5000 years ago up to ~2.5C warmer than the present.

Figure 9.

Mean air TJul reconstructions from Finnish Lapland: chironomid-based reconstruction from Lake Várddoaijávri (69°53′N, 26°31′E; 409 m asl) in northeastern Lapland, chironomid (Seppä et al., 2002) and pollen-based (Seppä and Birks, 2002) reconstructions from Lake Toskaljavri (69°12′N, 21°28′E; 704 m asl) and pollen-based reconstruction from Lake Tsuolbmajavri (68°41′N, 22°05′E; 526 m asl) (Seppä and Birks, 2001) in northwestern Lapland. A locally weighted scatterplot smooth with a span of 0.1 is applied to each of the mean air TJul reconstructions and fitted against the mean inferred temperatures in each of the cores (dashed horizontal lines). To estimate macrofossil-based paleotemperatures from Lake Njargajavri (69°46′N, 27°20′E; 355 m asl), Lake Korsajärvi (68°49′N, 22°05′E; 530 m asl), and Lake Kipojärvi (69°11′N, 27°17′E; 160 m asl), plant species distribution maps of Lampinen and Lahti (2013) were used to define the current area of distribution of several indicator species in Finland, and these distributions were compared to calculated mean air TJuldata (1961–2000) based on daily measurements by the Finnish Meteorological Institute (Venäläinen et al., 2005). The modern measured mean air TJuls for each site are marked with a cross.

Abstract

Paleoclimate reconstructions based on biological proxies present methodological challenges, especially during non-analog conditions, such as the early Holocene. Here, two chironomid-based training sets from Finland were amalgamated to create a more accurate transfer function of summer air temperature. The aim was to reconstruct Holocene paleoclimate in northernmost Lapland, in an area that has been either too warm or too cold for reliable reconstructions using the original calibration models. The results showed that the combined calibration model had improved performance statistics. The temperature trends inferred from the downcore chironomid record using the original and combined models were very similar. However, there were major changes in their absolute values with the combined model showing greatly improved accuracy. The chironomid-based temperature reconstruction showed significant correlation with the previous pollen-based reconstructions from northwestern Finnish Lapland. However, differences were observed in the temperature trends of the early Holocene, when the chironomid-inferred temperatures rapidly increased, but the pollen-based reconstructions lagged behind suggesting that a cool climate continued for much longer. However, similar to the chironomid record, new plant macrofossil evidence from northwestern Finland also showed warmer-than-present early Holocene temperatures. Therefore, we conclude that the early Holocene was probably warm in northern Lapland.